A gas turbine engine may be used to supply power to various types of vehicles and systems. For example, gas turbine engines may be used to supply propulsion power to an aircraft. Many gas turbine engines include at least three major sections, a compressor section, a combustor section, and a turbine section. The compressor section receives a flow of intake air and raises the pressure of this air to a relatively high level. In a multi-spool (e.g., multi-shaft) engine, the compressor section may include two or more compressors. The compressed air from the compressor section then enters the combustor section, where a ring of fuel nozzles injects a steady stream of fuel. The injected fuel is ignited by a burner, which significantly increases the energy of the compressed air.
The high-energy compressed air from the combustor section then flows into and through the turbine section, causing rotationally mounted turbine blades to rotate and generate energy. The air exiting the turbine section is then exhausted from the engine. Similar to the compressor section, in a multi-spool engine the turbine section may include a plurality of turbines. The energy generated in each of the turbines may be used to power other portions of the engine.
In addition to providing propulsion power, a gas turbine engine may also be used to supply either, or both, electrical and pneumatic power to the aircraft. For example, in the past some gas turbine engines include a bleed air port between the compressor section and the turbine section. The bleed air port allows some of the compressed air from the compressor section to be diverted away from the turbine section, and used for other functions such as, for example, main engine starting air, environmental control, and/or cabin pressure control. More recently, however, gas turbine engines are being designed to not include bleed air ports. This is in response to a desire to more fully utilize electrical power for main engine starting air, environmental control, and cabin pressure control. Thus, instead of using bleed air to support these various functions, the high pressure turbine may be used to drive one or more electrical generators to supply electrical power to support these functions.
The above-described configuration, in which the gas turbine engines drive one or more electrical generators, is believed to provide safe and reliable operations. However, it does present certain drawbacks. For example, during low power engine operations, such as during aircraft idle descent conditions, the high pressure turbine may need to supply a significant amount of energy to maintain the aircraft electrical load. This in turn can cause the low pressure turbine to run at an undesirably high speed, resulting in an undesirably high idle thrust. Although it has been postulated that this undesirable thrust can be alleviated by dumping a portion of the air discharged from the high pressure turbine overboard, such a solution results in wasted energy. In turn, this can significantly reduce engine efficiency, increase fuel consumption, and/or increase overall operational costs.
Hence, there is a need for a system that reduces the thrust generated by a “no-bleed-air” gas turbine engine during low power operations, that does not significantly reduce engine efficiency, and/or significantly increase fuel consumption, and/or increase overall operational costs. The present invention addresses one or more of these needs.